I didn't mean the FAQ link as a dig. I meant, honestly, that I figured you'd already read it, I just wanted to make sure we had that as the ground basis before moving on to other stuff.

Do the rear controls only control the volume of the headphone jacks, or do they control other stuff? Do they alter the volume of any of the main speakers?

If they're just volume controls for the headphone jacks, then they might have their own built-in headphone amplifier right there at the jack, and you only need to make sure they're fed with a proper signal to get them to work. But I don't know for sure, since I don't know anything about this particular system.

Ideally, you'd want to find an installer who's familiar with the system in that vehicle, or can at least obtain documentation on the system.